Dead Zones in the Marine Ecosystem

Hypoxic regions, also known as “dead zones”, are areas of the marine ecosystem characterised by a loss, or very low concentration, of oxygen. The number of dead zones worldwide has quadrupled over the last half century.[1] Over this period, many aquatic ecosystems have perished, and a large number of water dwelling creatures have felt the detrimental effects of human negligence. Unhealthy agricultural practices with over fertilisation and waste water release have been identified as the main contributors of this catastrophe.[2] The purpose of this article is to spotlight the causes and impacts of dead zones on ecosystems and provide future directions in preventing them.

When understanding a marine environment, it is fundamental to note that almost all organisms require oxygen. The availability of this resource plays an essential role on the distribution and survival of animals in aquatic environment.[3] Hypoxic regions, or dead zones, are areas of aquatic ecosystems -typically in the ocean, but occasionally in lakes- with very low concentrations of oxygen, not adequate to support life. From the mid-1900s, such occurrences have been nearly doubling every decade.[4] Scientists recently discovered the largest measured dead zone (Figure 1) since mapping began in 1985. This dead zone in the Gulf of Mexico covers nearly 8800 square miles, an area as big as the state of New Jersey, and has had devastating effects on the biodiversity of the region.[5]

Figure 1: The colossal dead zone (red colour) in the Gulf of Mexico. Credit: Donald Scavia.[6]

This continued decline in the oxygen concentration for more than half a century has hindered the survival fish and invertebrates and has impaired reproduction in certain aquatic species.[7][8] The process of deoxygenation due to natural causes, such as respiration and chemical reactions that consume oxygen, along with increased human activities have escalated the temperature and carbon dioxide level in the ocean.[5] This has altered the existence and distribution of marine species throughout the world.[9]

What is Causing Hypoxic Regions in Oceans?

In the 1960s, the number of dead zones worldwide was only about 50, however, today the number of dead zones exceeds 400 (Figure 2). As the exponential growth of dead zones prevails, the driving factor can be drawn to human causes.[12] The most apparent cause of hypoxia can be sourced to nutrient use in human practices. This can be from agricultural purposes, residential and industrial wastewater, fossil fuel burning, and the farming of leguminous plants.[13]

Through the means of various land runoff and wastewater release, fertilisation nutrients– specifically nitrogen, phosphorus, and potassium (NPK) – travel to their ocean destination.[12] Once situated, this runoff will stimulate the growth of algal blooms and other phytoplankton. This process is known as eutrophication, a process in which excessive nutrients in a body of water enhance plant growth to an extreme amount.[13]

Figure 2: Locations of dead zones throughout the world (indicated by red dots). Credit: Diya Das.[14]

As eutrophication continues to advance algal growth, a surplus of organic matter is synthesised. In turn, this promotes microbial growth as organic matter is essential for an optimal microorganism population.[9] However, such bacteria require both oxygen and organic matter to thrive. Thus, if increased levels of organic matter are entering the niche of these microorganisms, a substantial quantity of oxygen will also be exhausted.[2]

Put simply, due to the excessive use of NPK’s in agriculture and wastewater industries, oceans have had enhanced phytoplankton growth and increased microbial synthesis. Ultimately, this phytoplankton excessively consumes dissolved oxygen and leaves other marine dwelling creatures with insufficient oxygen for life processes. Therefore, it is the actions of various human applications in the modern world that has led to the creation of increased number of hypoxic regions.

Impact of Dead Zones on the Aquatic Ecosystem

One of the earliest effects of hypoxia starts with the inability of organisms to compensate for a lack of dissolved oxygen in an ecosystem. This, nearly always, results in an increase in water carbon dioxide levels. In most chemical processes, the reactant is used, and a product is produced. Likewise, as the oxygen content is depleted, the product of carbon dioxide accumulates in the environment. Furthermore, a net gain of carbon dioxide decreases the pH of the water and may cause acidity compaction for water-dwelling organisms. Acidosis, a buildup of acid in the bloodstream, is known to develop in the tissues of organisms that constantly breathe in carbon dioxide. Eventually, these conditions may lead to the death of an organism.[3]

Due to their unavailability of an essential resource, dead zones also cause habitat compression. [9] Many pelagic species depend on the cooler, deeper layer of the ocean during the warm climate of summer. However, hypoxia would potentially disallow the habituation of organisms in this location. As a simple consequence, more organisms are forced to a smaller habitat, which increases competition and reduces nursery habitat space. A prime example of this can be seen through the cod fish of the central Baltic region. Typically, cod lay eggs at the halocline, a vertical zone where salinity changes rapidly with depth.  Located between 70 and 80 meters in depth, the salinity is just enough to nurture the development of cod eggs. However, due to hypoxia, cod are prevented from entering their halocline, threatening the survival rate of cod offspring.[9]

Finally, hypoxia may alter the organisms in the population themselves. Deoxygenation may cause animals to have a range of physiological and behavioral effects. This includes decreased fitness, or offspring capabilities, and growth in predation.[11] With lower oxygen levels, only the species that have sufficient adaptation to hypoxia will continue to thrive. This will decrease the biodiversity within the community, thus altering the food chain and trophic levels in the ecosystem.[7]

Future Directions to Address This Problem

The first step is to understand that hypoxia is a serious issue that needs to be addressed at many levels. By measuring ecological footprints and monitoring the human impact on the marine environment, the specific causality for this threat can be diagnosed. It is evident from scientific studies[7][15] that this is a pressing global issue, which is increasing in scale.  The continued increase in the number of dead zones worldwide suggests that more action is necessary, and any steps currently being taken are not proving sufficiently effective to mitigate this problem.[16]

A specific solution would be devising an alternative eco-friendly fertiliser with lower levels of NPK’s. However, this eco-friendly fertiliser would have to be able to sustain the overall growth and productivity of the plant it is fertilising. The current research using organic materials with lower NPK ratios has shown promising results in reducing dead zones worldwide.[17][18] Technological advancement in agricultural practices could also play a role in the restoration of hypoxic regions.[7] Nevertheless, these solutions will only work if society grows to view the marine ecosystem with greater care and concern.


  1. Damian Carrington, “Oceans suffocating as huge dead zones quadruple since 1950, scientists warn”, The Guardian, January 4, 2018, https://www.theguardian.com/environment/2018/jan/04/oceans-suffocating-dead-zones-oxygen-starved
  2. Cheryl Lyn Dybas, “Dead Zones Spreading in World Oceans”, BioScience 55, no. 7 (July 1, 2005): 552-557, https://doi.org/10.1641/0006-3568(2005)055[0552:DZSIWO]2.0.CO;2.
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  6. Donald Scavia, “Dead zones are a global water pollution challenge – but with sustained effort they can come back to life”, The Conversation, May 4, 2018, http://theconversation.com/dead-zones-are-a-global-water-pollution-challenge-but-with-sustained-effort-they-can-come-back-to-life-96077.
  7. Denise Breitburg, Lisa A. Levin, Andreas Oschlies, Marilaure Gregoire, Francisco P. Chavez, Daniel J. Conley, Véronique Garçon et al.,Declining oxygen in the global ocean and coastal waters”, Science 359, no. 6371 (January 5, 2018): 1-11, https://doi.org/10.1126/science.aam7240.
  8. Donald F. Boesch, Walter R. Boynton, Larry B. Crowder, Robert J. Diaz, Robert W. Howarth, Laurence D. Mee, Scott W. Nixon et al., “Nutrient Enrichment Drives Gulf of Mexico Hypoxia”, Eos, Transactions, American Geophysical Union 90, no. 14 (April 7, 2009): 117-18, https://doi.org/10.1029/2009EO140001.
  9. Robert J. Diaz and Rutger Rosenberg, “Spreading Dead Zones and Consequences for Marine Ecosystems”, Science 321, no. 5891 (August 15, 2008): 926-29, https://doi.org/10.1126/science.1156401.
  10. Bruce A. Babcock and Catherine L. Kling, “Costs and Benefits of Fixing Gulf Hypoxia”, Iowa Ag Review 14, 4(2015): 7-10. http://lib.dr.iastate.edu/iowaagreview/vol14/iss4/4.
  11. Sergey S. Rabotyagov, Catherine L. Kling, Philip W. Gassman, Nancy N. Rabalais and Robert E. Turner, “The Economics of Dead Zones: Causes, Impacts, Policy Challenges, and a Model of the Gulf of Mexico Hypoxic Zone”, Review of Environmental Economics and Policy 8, no. 1 (January 4, 2014): 58-79, https://doi.org/10.1093/reep/ret024.
  12. Nancy N. Rabalais, Wei-Jun Cai, Jacob Carstensen, Daniel J. Conley, Brian Fry, Xinping Hu, Zoraida Quiñones-Rivera et al., “Eutrophication-driven deoxygenation in the coastal ocean”, Oceanography 27, no. 1 (2014): 172-83, https://doi.org/10.5670/oceanog.2014.21.
  13. Denise Breitburg, Grégoire Marilaure and Kirsten Isensee, “The Ocean is losing its breath: declining oxygen in the world’s ocean and coastal waters; summary for policy makers”, Intergovernmental Oceanographic Commission of UNESCO, 2018, 1-39, https://unesdoc.unesco.org/ark:/48223/pf0000265196?posInSet=1&queryId=N-EXPLORE-3494e7f6-0a71-439a-b3a3-354e5fe50adc.
  14. Diya Das, “Dying Oceans”, Down to Earth, last modified August 18, 2015, https://www.downtoearth.org.in/coverage/environment/dying-oceans-50679.
  15. J. Zhang, D. Gilbert, A. Gooday, L. Levin, S. W. A. Naqvi, J. J. Middelburg, M. Scranton et al., “Natural and human-induced hypoxia and consequences for coastal areas: Synthesis and future development”, Biogeosciences 7, (May 10, 2010): 14431467, https://doi.org/10.5194/bg-7-1443-2010.
  16. Aaron M. Cohen, “Oceans’ dead zones on the rise”, The Futurist 43, no. 6 (Nov.-Dec. 2009): 7+, http://link.galegroup.com/apps/doc/A210614259/AONE?u=miam50083&sid=AONE&xid=3360eae9.
  17. A. F. Bouwman, G. Van Drecht, J. M. Knoop, A. H. W. Beusen and C. R. Meinardi, “Exploring changes in river nitrogen export to the world’s oceans”, Global Biogeochemical Cycles 19, no. 1 (March 2005): 1002-1114, https://doi.org/10.1029/2004GB002314.
  18. Ellery Ingall and Richard Jahnke, “Evidence for enhanced phosphorus regeneration from marine sediments overlain by oxygen depleted waters”, Geochimica et Cosmochimica Acta 58, no. 11 (June 1994): 2571-2575, https://doi.org/10.1016/0016-7037(94)90033-7.

About the Author

Atiksh Chandra, USA

Atiksh is an 8th grade student attending Falcon Cove Middle School in Weston, Florida. Biology is one of his favourite subjects. He enjoys learning about different organisms and their impact on our surroundings. His other hobbies include public speaking and playing the piano.

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